Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) pulsed optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals and reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
The performance of a QKD system can be degraded by noise in the form of photons generated by three different mechanisms. The first is forward Raman scattering, in which frequency-shifted photons are generated and co-propagate with the quantum signal photons. Raman scattering in an optical fiber limits the power that can be put into a single fiber because of a transfer of energy from a high power signal to the single-photon wavelength.
The second mechanism is Raman backscattering, in which frequency-shifted photons are generated and propagate in the opposite direction to the quantum signal photons.
The third mechanism is Rayleigh scattering, in which photons are elastically scattered back in the opposite direction of the quantum signal photons.
The scattering of light in an optical fiber—and particular forward Raman scattering—is problematic in multiplexing the different channels of a QKD system because of the noise it creates in the detection process.
Two simple solutions have been proposed to overcome the effects of light scattering in combining different channels onto a single optical fiber. The first solution is to use one fiber for the public discussion channel, possibly the sync channel as well, and a second fiber for the quantum channel. The second solution is to limit the fiber length so that the input power can be reduced, and so the scattering power transfer ratio is lower with the shorter distance. Both of these solutions, while simple, are also unappealing because they are not particularly robust and are ill-suited for a commercially viable QKD system.
The prior art relating to multiplexing the different channels associated with QKD includes U.S. Pat. No. 6,438,234 (“the '234 patent”). In the '234 patent, the sync signal is time-multiplexed with the quantum channel. The prior art also includes U.S. Pat. No. 5,675,648 (“the '648 patent). The '648 patent proposes the idea of having a “common transmission medium” (i.e., an optical fiber) for the quantum channel and the public channel, where the public channel also carries a calibration signal.
However, the prior art does not address the daunting problem of combining the relatively strong public and sync channels with the very weak quantum channel. In particular, the '648 patent does not address how the public channel can be multiplexed with the quantum channel in the “common transmission medium” in a way that will not interfere with detecting the single-photons associated with the quantum channel.
Also, in the '234 patent, a sample-and-hold type of phase lock loop needs to be implemented to hold the sync timing while working on single photons. However, the difficulties of multiplexing sync and quantum channel are less challenging than the task of multiplexing the public (data) channel and the quantum channel. The '234 patent does not address the issue of transmitting the public channel and the quantum channel over the same optical fiber.
The publication “Eighty kilometer transmission experiment using an InGaAs/InP SPAD-based quantum cryptography receiver operating at 1.55 um” by P. A. Hiskett, G. Bonfrate, G. S. Buller, and P. D. Townsend, published in the Journal of Modern Optics, 2001, vol. 48, no. 13, pp. 1957-1966, suggests an approach to combining the sync and quantum channels. The light from the transmitter laser is split into a quantum signal and a sync signal. The sync signal is sent over a separate fiber and upon entering the receiver is amplified by an erbium doped fiber amplifier (EDFA). After the amplified light signal is converted into electrical signal, the electrical signal is used to gate the receiver's detector.
It would be desirable to wavelength-multiplex 10 MHz Ethernet public discussion traffic (i.e., the public channel) onto the same fiber as the sync and quantum channels. However, the optical power of the Ethernet public channel signal must be significantly reduced to prevent scattering and other such interference that reduces the ability to detect the channels. Unfortunately, reducing the public channel power results in an unacceptably low signal-to-noise ratio for the public channel for any QKD system with a satisfactory distance or span. While the use of an optical fiber amplifier (e.g., an erbium-doped fiber amplifier or EDFA) can increase the amplitude of the optical signal and remove the need for a narrow band optical filter, its output will still have a very low signal to noise ratio.